The present invention relates to a process for producing a pitch-based carbon fiber.
It is known that a carbon fiber having high strength and high elastic modulus can be produced by heat-treating pitch to form a mesophase, then melt-spinning the pitch and subjecting the resulting pitch fiber to infusiblization, carbonization and graphitization (see Japanese Patent Publication Nos. 4286/1985 and 3567/1984). By this known method there can be obtained a carbon fiber having a tensile strength of 100 to 200 kg/mm.sub.2 and an elastic modulus of 20 to 70 ton/mm.sub.2.
If a carbonized fiber obtained using mesophase pitch is calcined in a graphitization region of 2,500.degree. to 3,000.degree. C., a graphitized structure is developed and the elastic modulus increases with increase of the calcining temperature. That is, an interlayer spacing (d.sub.002) which can be said to be a measure of graphitization becomes narrower with increase of the calcining temperature, and it has been reported that when calcination is made in the graphitization region, the value of d.sub.002 becomes 3.37 .ANG. or smaller (see Japanese Patent Laid-Open No. 19127/1974). And the graphitized fiber has a three-dimensional order of a polycrystalline graphite characterized by the presence of (112) crosslattice line and (100), (101) lines in an X-ray diffraction pattern thereof.
On the other hand, with development of the graphitized structure, there occurs shrinkage of the carbon layer surface, causing cracks in the fiber. The cracks cause deterioration in the mechanical strength of the fiber. The carbonized fiber produced from mesophase pitch is formed by a giant domain (a carbon layer having a hexagonal network structure of carbon) extending straight in the fiber axis direction, so when graphitized, it is easily cracked.
Therefore, although a carbon fiber of high elastic modulus is obtained from pitch, it has been difficult to obtain a carbon fiber having a higher strength (e.g. 250 kg/mm.sup.2 or more).